Power boats, especially of the inboard type, are often equipped with trim tabs which extend from the rear of the hull and are pivotably attached at a point near the bottom surface of the boat. These tabs are adjustable to permit trimming the craft in view of varying load distributions, speeds, water and wind conditions and the like. Usually a hydraulic cylinder is used for tab positioning and is mounted with one end attached to the boat transom and the other, the rod end, to the trim tab for positioning. Rod extension and retraction is by a hydraulic control unit mounted within the boat and controlled by the operator Examples of such trim tab and cylinder installations are shown and described in U.S. Pat. Nos. 3,628,487 and 4,854,259.
Several design parameters affect the configuration of such hydraulic cylinders. One is that the cylinder must be adapted for easy, quick repair since downtime represents at least an annoyance to the boat operator. If the boat is used for charter or other commercial service, such downtime can have a serious impact on profitability. If the cylinder is configured so that parts are easily lost during disassembly, downtime may be lengthened appreciably. In addition, a cylinder for adjusting trim tab position must be capable of being mounted for pivoting movement since such movement is required as the position of the tab is changed.
The ideal hydraulic cylinder for trim tab adjustment would not only have design features which would permit it to be manufactured for a lower cost but would also be configured to permit very easy repair while yet minimizing the risk of loss of parts. The ideal cylinder would also be readily adapted to be mounted for pivoting movement.
One example of a hydraulic cylinder which is arranged for pivot mounting is shown in U.S. Pat. No. 2,790,426. The cylinder shown therein can neither be installed nor removed from its mounting base without disturbing at least one support block. In addition, both the outer and inner cylinder walls include portions which are machined and which are therefore more expensive to manufacture. And when the cylinder gland is removed, the outer wall (which is a part least likely to require service) immediately becomes separated from the trunnion. If service is being performed over water, this wall could be readily and irretrievably lost.
Examples of hydraulic swivel joints are shown and described in U.S. Pat. Nos. 3,981,329 and 2,768,842. Neither of the swivel joints described in those patents permit removal of the center journal block without dismantling and removing part of the joint components.
A double wall hydraulic cylinder which is designed to minimize the amount of machining required in its manufacture and which is readily serviceable or removable from a boat transom without disturbing the mounting lugs or hydraulic conductors while yet minimizing the risk of loss of parts would be a distinct advance in the art.